Fuse and disconnect device for manual or thermal operation



Jan. 17, 1967 M. G. LEONARD FUSE AND DISCONNECT DEVICE FOR MANUAL OR THERMAL OPERATION Filed Feb. 26, 1965 INVENTOR Merrill G. Leonard BYkWW ATTORNEY 3 kahuna 9 m mm a WITNESSES JZmzm 5W United States Patent M 3,299,235 FUSE AND DISCONNECT DEVICE FOR MANUAL 0R THERMAL OPERATION Merrill G. Leonard, Brookfield Township, Fowler, Ohio, 5

This invention relates to combination fuse and disconnect devices which may open the circuit in response to a temperature rise or may be manually broken, and more particularly to a combination fuse and disconnect device suitable for mounting inside a high voltage bushing.

It is frequently desirable in connection with electrical equipment to control the circuit either manually or automatically, such for example as the circuit of a pad mounted transformer. In the past it has been customary to utilize a switch or a circuit breaker, and to provide a fuse separate from the switch or circuit breaker and mounted at a suitable position with respect to the transformer. In a copendin-g application of Merrill G. Leonard and Leonard L. Wright for a Fuse and Disconnect Device for Mounting Inside a High Voltage Bushing, Ser. No. 403,105, filed Oct. 12, 1964, and assigned to the assignee of the instant invention, there is described and claimed a tubular structure remova-bly mounted in a bore in a high voltage bushing and which contains a fusible element and a mechanical break device for rupturing the fusible element when it is desired to open the circuit manually.

My invention constitutes a further advance in the fuse and disconnection art. In summary, a tube-like structure which contains a fuse is inserted in a bore in a high voltage bushing, the bushing being mounted, for example, in a transformer tank. The outer end of the tubular structure is threaded on the external surface thereof, to form a ratch-like surface and the outer end of the high voltage bushing has a pawl so arranged that the tubular structure may be inserted by a single stroke into the bore in the bushing, the threads of the ratch-like surface passing freely over the pawl, whereas to Withdraw the tubular structure from the bore in the bushing, it is necessary to turn the tubular structure, the threads being engaged by the pawl during the withdrawal process. A connecting rod extends into the tubular structure and has one end of a fuse Wire connected to the inner end thereof, the other end of the fuse wire looping over the adjacent end of the tubular structure, which is composed of insulating material, and making electrical connection with a collar composed of conductive material mounted on the outer surface of the tubular structure at the inner end of the structure. When the tubular structure is inserted in the bore in the high volage bushing, the collar makes electrical contact with a spring contact finger mounted on the inner end of the bushing to establish an electrical circuit, but the electrical circuit is not made until at least one thread of the ratch-like surface has passed the pawl so that the tubular structure is in effect locked into the bore in the bushing before the electrical circuit is made. The aforementioned connecting rod may be partially withdrawn to break the fuse and interrupt the circuit, and only after the connecting rod has been withdrawn to a position whereat the fuse wire is broken is the tubular structure threadedly removable from the bore, a pin in the end of the high voltage bushing, cooperating with means carried by the connecting rod, retaining the tubular structure against angular or rotary motion until the connecting rod has been partially withdrawn and the electrical circuit interrupted. When the connecting rod has been withdrawn to the circuitbreaking position, a pin carried by the connecting rod en- 3,299,235 Patented Jan. 17, 1967 gages a slot in the tubular structure, so that turning force applied to the connecting rod turns the tubular structure. Accordingly it is seen that my fuse and disconnect device offers maximum convenience in inserting it into the bore in the bushing and withdrawing it therefrom, and offers maximum safety when breaking the circuit as well as maximum safety when remaking the electrical circuit after a new fuse wire has been attached.

Accordingly, a primary object of the invention is to provide a new and improved fuse and disconnect device.

Another object is to provide a new and improved fuse and disconnect device suitable for manual or thermal operation.

A further object is to provide a new and improved manually operable disconnect device suitable for mounting in a bore in a bushing and which cannot be removed from the bore in the bushing until the electrical circuit therethrough has been broken.

A further object of the invention is to provide a new and improved fuse and disconnect device suitable for mounting in a bore in a bushing, and which can be thrust into the bore by a single stroke, the electrical circuit not being completed until the tubular structure is effectively locked in place in the bore in the bushing.

These :and other objects will become more clearly apparent after a study of the following specification, when read in connection with the accompanying drawing, in which the single figure thereof shows a cross-section through the bushing and the tubular fuse and disconnect structure according to the preferred embodiment of my invention.

In the figure, the reference numeral 10 generally indicates a high voltage bushing composed of a suitable insulating material such as porcelain, disposed in an opening 11 in a transformer tank 12 with the gasket 13 provided for sealing the opening. The bushing 10 is seen to have a bore 15 extending therethrough, the bore 15 having at the inner end thereof a slot 16 to permit free movement therein of a spring contact member 17 held in place by bolt 18 passing through bore 19 with the nut 20 retaining the bolt 18 securely in place, and having lead 21 connected thereto to form part of an electrical circuit.

The other or outer end of the bore 15 has a portion of enlarged diameter 24. Disposed securely within the bore portion 24 of enlarged diameter is a metallic housing 28 generally cylindrical in shape having an axial bore 29 the-rethrou-gh. Cement 25 may hold housing 28 securely in place. The entrance end of the portion of enlarged diameter has a larger annular groove 26 opening into the top of the bushing, for purposes to be apparent hereinafter. The bore 29 is seen to be of smaller diameter than the main portion of the bore 15 for reasons which will become hereinafter more clearly apparent. The cylindrical housing 28 has an outwardly extending annular flange portion 30 which has at a predetermined position therein a slot 31 in which is mounted a pawl 32 for rotation on pin 33. Spring biasing means, not shown for convenience of illustration, exerts a force on the pawl, pressing the pawl against the tubular structure generally designed 35 having the threaded end 36 forming a ratch-like surface. It is further seen that the flange portion 30 has a bore 38 therein in which is disposed a pin 39, for purposes to become hereinafter more clearly apparent.

The aforementioned tubular structure generally designated 35 is, in the embodiment shown, composed of two sections, an upper section 34 having the aforementioned threaded end 36 and composed of metal, and a lower section generally designated 37 which is composed of insulating material. The upper end of the insulating wire 49 is intact.

tubular section 37 is internally threaded at 42 as shown, and the lower end of the metallic section 34 is of reduced diameter and threaded at 43, so that the metallic section 34 and the insulating section 37 may be threadedly secured together. The shoulder 44 formed at the position on the portion 34 where the diameter is reduced abuts against the adjacent end of the insulating portion 37.

The lower end of the insulating tubular portion 37 is seen to be of reduced diameter at 46 and to have threads 47 on the external surface thereof. Disposed on this end of the tubular structure 37 is an internally threaded collar 48 composed of conductive material. Collar 48 makes contact with the aforementioned spring contact 17 while the tubular structure is fully inserted in the bore in the bushing. The conductive collar 48 has one end of fuse wire 49 secured thereto.

Disposed inside the central bore 52 of the tubular struc ture generally designated 35 and extending all the way through the metallic portion 34 and partially into the insulating portion 37, and slidable within limits therein, is a pull rod or connecting rod generally designated 54 composed of conductive material. The connecting rod 54 has the end cut away to form a flat surface 55 in which is threadedly disposed a screw 56 which attaches the upper end of the fuse wire 49 to the connecting rod. Near the lower end of the rounded portion of the connecting rod is a pin 57 adapted to move into a slot 58 in the extended end portion 59 of metallic portion 34 when the pin and slot are aligned and the connecting rod drawn upward, for purposes to be made hereinafter more clearly apparent.

It will be seen then that the connecting rod 54 can move within the bore in the tubular structure a distance from a first position corresponding to that shown to a second position whereat the pin 57 is engaged by the upper end of the slot 58.

The lower end 59 of the metallic portion 34 of the tubular structure has an additional slot on the opposite side thereof corresponding to the slot 58, the additional slot not being shown for convenience of illustration, and it will be understood that the pull rod 54 also has an additional pin, not shown, oppositely disposed to the pin 57.

Attached to the upper end of the pull rod or connecting rod 54 is a flange 65 having a downwardly extending annular lip portion 66 and having an extended'handle portion 67 with eye 68. Flange 65 has slot 64 therein.

The aforementioned metallic portion 34 of the tubular structure generally designated 35 is seen to have an outwardly extending annular flange portion 62 at the upper end thereof, in which is mounted an upwardly extending pin 63. The lower surface of the flange portion 62 abuts against the adjacent surface of the flange portion 30. Pin 63 normally passes through and is engaged by slot 64.

The aforementioned pin 39 on housing 28 also normally passes through and is engaged by slot 64. It is noted that pin 39 extends considerably farther than pin 63.

It will be understood that electrical connection is made to the handle portion 67 of the pull rod as by a hot line clamp, the circuit being completed from lead 21 through the fuse wire 49 through pull rod 54 to handle 67, the electrical connection to handle 67 being symbol ized by the lead 69.

In the operation of the apparatus, let it be assumed for purposes of illustration that the circuit between leads 21 and 69 is complete, that the pull rod generally designated 54 is in the position shown, and that the fuse Then let it be assumed that for some reason, such for example as a fault in the circuit, it becomes desirable to interrupt the electrical circuit. The eye 68 is grasped as by a hook stick and moved upwardly, breaking the fuse wire 49 and interrupting the circuit. The flange 65 as noted has the slot 64 therein which retains the flange 65 and pull rod 54 against angular or rotary movement with respect to the high voltage bushing 10 while pin 39 is extending through the slot 64. The movement of the eye 68 and connecting rod 54 upwardly, in addition to freeing the flange 65 and connecting rod for rotary motion with respect to the bore in the high voltage bushing, moves pin 57 (and the corresponding pin on the opposite side of the rod) into the slot 48, so that by turning eye 68 and connecting rod 54 in the appropriate angular direction, in this case a counterclockwise direction looking into the flange 65-eye 68 end of the device, rotary force is transferred from the connecting rod 54 to the tubular structure 35, and the pull rod 54, carrying with it the tubular structure 35, is removed from the bore in the high voltage bushing as the threads 36 on the ratch surface at the upper end of the tubular structure turn with respect to the pawl 32.

Pin 63 in groove 64 maintains pin 57 aligned with groove 58.

After the entire tubular structure has been removed from the bushing, the conducting and insulating portions 34 and 37 may be disconnected from each other at threads 42-43. A new fuse wire may be inserted at the end of the pull rod between screw 56 and the collar 48 and the tubular sections may be thereafter rejoined at 42-43. Grasping the tubular structure and pull rod by the eye 68, the whole tubular structure may be thereafter thrust into the bore in the high voltage bushing, the threads 36 passing freely past the pawl 32 when they are moving in a downward direction, as shown in the figure.

As previously stated, in the process of thrusting the tubular structure 35 into place, the pawl runs over the threads in the manner of a ratchet, so that no rotation is necessary. Sufficient thread length is provided so that the pawl will engage at least one thread before electrical contact is made at the inner end of the bushing between the spring contact 17 and collar 48.

In order to insure the rotation of the tubular structure 35 with the connecting rod 54, as previously stated a.

slot 58 is provided internally so that the pin 57 and slot 58 will mechanically engage when the connecting rod is pulled out. On the other hand, it is desirable that the operator of the device should not be able to unscrew the eye before the fuse wire is broken, and for this purpose the pin 39 is provided which restricts rotation of the eye at the flange until the connecting rod has been pulled out to a position whereat the fuse wire is broken. If desired, some small further outward movement of the connecting rod may be required before pin 57 engages slot 58. The pin 63 is made long enough so that the eye 68 will not go back in place until the slot 58 and pin 57 are properly aligned, and if the wrong angular position is tried the circuit will not be completed between 17-48 at the inner end of the bushing.

Pin 63 is also used to maintain alignment between the tube 35 and the fuse link, so that the pin 57 will engage slot 58 when the connecting rod is first pulled upward in the tubular structure.

The portion of the slot 64 which engages pin 39 may be considerably wider than the slot portion which engages pin 63.

It is seen, then that the apparatus is ably suited to fulfill the aforedescribed objects of the invention. The special thread and pawl, which provide for ratchet engagement and screw-threaded disengagement make provision for maximum safety in the operation of the manually and thermally operable protective fuse link. Furthermore, the tubular structure is locked in before the circuit is made, and the circuit is broken before the tubular structure is unlocked.

Whereas I have shown and described the invention with respect to an embodiment thereof which gives satisfactory results, it should be understood that changes may 6 be made and the equivalents substituted without departing from the spirit and scope of the invention.

I claim as my invention:

1. In fuse and disconnect apparatus, in combination, a bushing composed of insulating material and having an axial bore therethrough, the bushing having electrical contact means at one end thereof, a pawl disposed at the other end of the bushing, a tubular structure composed at least partially of insulating material slidably disposed in the bore in the bushing and carrying other contact means at one end thereof which makes electrical connection with the contact means on the bushing While the tubular structure is in the bore in the bushing, the tubular structure having a plurality of threads on the outer surface thereof at the other end thereof adapted to be engaged by said pawl, connecting rod means composed of conductive material slidably disposed in the tubular structure, first coupling means on the connecting rod means, the tubular structure including second coupling means, the first and second coupling means engaging each other while the tubular structure and connecting rod means have a predetermined axial relationship with respect to each other for transmitting rotary motion from the connecting rod means to the tubular structure, and a fuse element extending between the other contact means and the adjacent end of the connecting rod means, said threads being slidable past the pawl when the tubular structure is inserted in the bore in the bushing, the pawl engaging the threads when the tubular structure is being withdrawn from the bore in the bushing whereby the tubular structure must be turned to permit movement of the threads past the pawl.

2. Fuse and disconnect apparatus according to claim 1, in which the first coupling means is additionally characterized as including at least one pin extending from the rod means near the end at which the fuse element is attached, and in which the second coupling means is additionally characterized as including at least one slot in the tubular structure adapted to be engaged by said pin after the connecting rod means has been moved a predetermined distance whereby rotary forces thereafter applied to the connecting rod means rotate the tubular structure to move the threads past said pawl.

3. In fuse and disconnect apparatus, in combination, a high voltage bushing having an axial bore extending the length thereof, the bushing having electrical contact means disposed on one end thereof, a tubular structure composed at least partially of insulating material slidably disposed in the bore in the bushing, other contact means mounted on one end of the tubular structure and normally making electrical connection with the contact means mounted on the bushing, the tubular structure having a plurality of axially spaced annular threads on the outer wall at the other end thereof to form a ratch-like surface, connecting rod means composed of conductive material slidably disposed in the tubular structure, fuse means connected between the connecting rod means and the other contact means, means attached to the connecting rod means for withdrawing the connecting rod means partially from the tubular structure to break the fuse means, a pawl disposed at the other end of the high voltage bushing normally engaging one of the threads of the ratch-like surface and retaining the tubular structure in the bore in the high voltage bushing, and means carried by the high voltage bushing and cooperating with the withdrawing means for preventing the withdrawal of the tubular structure by turning the threads past the pawl until the connecting rod means has been partially withdrawn from the tubular structure and the fuse means broken.

4. In fuse and disconnect apparatus, in combination, a high voltage bushing having an axial bore extending the length thereof, the bushing having electrical contact means disposed at one end there-of, a tubular structure composed at least partially of insulating material slidably disposed in the bore in the bushing, other contact means mounted on one end of the tubular structure and normally making electrical connection with the contact means mounted on the bushing, the tubular structure having a plurality of axially spaced annular threads on the outer wall at the other end thereof to form a ratch-like surface, connecting rod means composed of conductive material slidably disposed in the tubular structure, the connecting rod means having at least one pin at .a predetermined position thereon, the tubular structure having at least one slot at a predetermined position therein adapted to engage said pin When the connecting rod means has been moved to a predetermined position whereby rotary movement of the connecting rod means carries the tubular structure therewith, fuse means connected between the connecting rod means and the other contact means, the connecting rod means having handle means for withdrawing the connecting rod means partially from the tubular structure to break the fuse means, the connecting rod means having an outwardly flaring annular flange with a slot therein, the tubular structure having a pin at the end thereof adapted to normally pass through said last-named slot, latch means on the bushing including a pawl normally engaging one of the threads and retaining the tubular structure in the bore in the high voltage bushing, the last-named pin carried by the tubular structure cooperating with the slot in the flange to maintain the first-named pin in substantial alignment with the first-named slot whereby the first-named pin readily engages the first-named slot when the connecting rod means is partially withdrawn and rotary force applied to the connecting rod means is readily transferred to the tubular structure.

5. Fuse and disconnect apparatus comprising, in combination, a high voltage bushing adapted to be mounted in the aperture of a tank enclosing electrical apparatus and having a bore extending axially thereof, a tubular structure removably mounted in the bore in the bushing and comprising an inner end portion composed of insulating material and an outer end portion composed of conductive material, contact-means carried by the end portion of insulating material, other contact means on the bushing normally making electrical connection with the contact means of the tubular structure, the outer end of the tubular structure being threaded on the outer surface thereof, pawl means mounted at the adjacent end of the bushing and adapted to engage the threads, a connecting rod composed of conductive material slidably disposed in the tubular structure, a fuse wire connected between the inner end of the connecting rod and the contact means on the tubular structure, the tubular structure and the connecting rod being constructed and arranged whereby when the connecting rod is moved to a predetermined position within the tubular structure angular force applied to the connecting rod is transmitted to the tubular structure whereby the tubular structure has the threads thereof turned past the pawl means, the tubular structure upon being rein serted in the bore in the bushing after a new fuse wire has been attached not resulting in an electrical circuit between the contact means and the other contact means until the tubular structure has moved far enough into the bore in the bushing so that the pawl means engages one of the threads on the outer surface of the tubular structure and locks the tubular structure in the bore in the bushing.

, movably mounted in a bore in a bushing composed of insulating material, the bushing having electrical contact means at one end thereof, in combination, a tubular structure composed at least partially of insulating material slidably disposed in the bore in the bushing and carrying other cont-act means which makes electrical connection with the contact means on the bushing while the tubular structure is in the bore of the bushing, connecting rod means composed of conductive material slidably disposed in the tubular structure and movable from a normal first position to a second position therein, means carried by the connecting rod means for engaging the tubular structure when the connecting rod means has moved to said second position whereby rotary forces applied to the connecting rod means are transmitted to the tubular structure, the tubular structure having threads on the outer surface thereof at the end thereof opposite to the end carrying the other contact means, the bushing having a pawl mounted at the end thereof adjacent the threads and normally engaging said threads, and a fuse element extending between the other contact means and the adjacent end of the connecting rod means while the rod means is in the first position, moving the connecting rod means to the second position breaking the fuse element and interrupting the electrical circuit, the connecting rod means and tubular structure being adapted to be thereafter rotated to move said threads past said pawl whereupon the connecting rod means and tubular structure may be withdrawn as a unit from the bore in the bushing.

8. In fuse and disconnect apparatus adapted to be removably mounted in a bore in a bushing composed of insulating material, the bushing having electrical contact means at one end thereof, in combination, a tubular structure composed at least partially of insulating material slidably disposed in the bore in the bushing and carrying other contact means which makes electrical connection with the contact means on the bushing while the tubular structure is in the bore of the bushing, connecting rod means composed of conductive material slidably disposed in the tubular structure and movable from a first position to a second position therein, means carried by the connecting rod means for engaging the tubular structure when the connecting rod means has been moved to the second position whereby rotary movement of the connecting rod means carries with it the tubular structure, a pin mounted on the end of the bushing, the connecting rod means having an outwardly extending flange with a slot therein, the pin normally engaging the slot while the connecting rod means is in the first position and preventing rotary movement of the connecting rod means with respect to the bushing, partial withdrawal of the connecting rod means to the second position freeing the pin from the slot and permitting turning movement of the connecting rod means, and a fuse element extending between the other contact means and the adjacent end of the connecting rod means while the rod means is in the first position, moving the connecting rod means to the second position breaking the fuse element and interrupting the electrical circuit, the tubular structure having threads on the outer surface thereof at the end thereof near said pin, the bushing having a pawl mounted at the adjacent end thereof normally engaging the threads, rotary motion applied to the tubular structure by way of the connecting rod means turning the threads past the pawl whereafter the connecting rod means and the tubular structure may be withdrawn as a unit from the bore in the bushing.

9. In fuse and disconnect apparatus, in combination, a bushing composed of insulating material and having an axial bore therethrough, the bushing having electrical contact means at one end thereof, a tubular structure composed at least partially of insulating material slidably disposed in the bore in the bushing and carrying other contact means which makes electrical connection with the contact means on the bushing while the tubular structure 7 other while the tubular structure and connecting rod means have a predetermined axial relationship with respect to each other for transmitting rotary motion from the connecting rod means to the tubular structure, a fuse element extending between the other contact means and the adjacent end of the connecting rod means, and releasable retaining means for retaining the tubular structure and connecting rod means within the bore, the retaining means being adapted to be released by rotating the connecting rod means and tubular structure in the bore in the bushing.

10. Fuse and disconnect apparatus according to claim 9 in which the releasable retaining means is additionally characterized as including threads on the end of the tubular structure opposite to the end carrying the other contact means, and a pawl mounted in the adjacent end of the bushing and normally engaging the threads.

11. Apparatus according to claim 9 wherein the releasable retaining means is additionally characterized as including threads on the outer surface of the end of the tubular structure opposite to the end carrying the other contact means, and a pawl mounted in the bushing normally engaging the threads, the retaining means being released by turning the threads on the tubular structure past the pawl whereafte-r the rod and tubular structure may be removed as a unit from the bore in the bushing, the threads and pawl being so constructed in relation to each other that the tubular structure may be thrust into the bore in the bushingby a single movement, the pawl sliding freely over the threads while the tubular structure is moving in a direction toward the interior of the bushing.

12. Apparatus according to claim 9 wherein the bushing is additionally characterized as having a pin thereon and the connecting rod means as additionally characterized as having a flange with a slot therein normally engaging the pin, the retaining means is additionally characterized as including threads on the exterior surface of the end of the tubular structure opposite to the end carrying the other contact means, and a pawl mounted on the bushing normally engaging the threads, said pin While in said slot preventing rotary movement of the connecting rod means in the tubular structure whereby the tubular structure may not be withdrawn past the .pawl until the connecting rod means has been withdrawn to a position where the pin is disengaged from the slot.

13. In fuse and disconnect apparatus adapted to be removably mounted in a bore in a bushing composed of insulating material, the bushing having electrical contact means at one end thereof, in combination, a tubular structure composed at least partially of insulating material slidably disposed in the bore in the bushing and carrying other contact means which makes electrical connection with the contact means on thebushing while the tubular structure is in the bore in the bushing, connecting rod means composed of conductive material disposed in the tubular structure and slidable from a first position to a second position therein, a fuse element extending between the other contact means and the adjacent end of the connecting rod means while the connecting rod means is in the first'position, movement of the connecting rod means to the second position breaking the fuse element, means carried by the connecting rod means for engaging the tubular structure when the connecting rod means has moved to the second position whereby rotary forces applied to the connecting rod means are transmitted to the tubular structure, the tubular structure having threads on the external surface thereof at the end thereof opposite to the end carrying the other contact means, a pawl mounted in the bushing at the end there-of adjacent the threads and normally engaging the threads, rotation of the tubular structure moving the threads past the pawl whereafte-r the tubular struc ture and connecting rod means may be withdrawn as a unit from the bore in the bushing, the connecting rod means including flange means having a slot therein, a pin mounted on the bushing and normally engaging the slot to prevent rotation of the connecting rod means until the connecting rod means has been withdrawn to a position whereat the pin is freed from the slot, said last-named position corresponding to the second position whereat the fuse element is broken, whereby the tubular structure may not be withdrawn from the bore in the bushing until the electrical circuit therethrough has been interrupted.

14. Apparatus according to claim 13 in which the tubular structure is additionally characterized as having an additional .pin passing through the slot in the flange of the connecting rod means, the additional pin substantially maintaining the alignment of the means carried by the connecting rod means which engages the tubular structure when the connecting rod means has been withdrawn to the second position.

References Cited by the Examiner UNITED STATES PATENTS BERNARD A. GILHEANY, Primary Examiner. H. GILSON, Assistant Examiner. 

1. IN FUSE AND DISCONNECT APPARATUS, IN COMBINATION, A BUSHING COMPOSED OF INSULATING MATERIAL AND HAVING AN AXIAL BORE THERETHROUGH, THE BUSHING HAVING ELECTRICAL CONTACT MEANS AT ONE END THEREOF, A PAWL DISPOSED AT THE OTHER END OF THE BUSHING, A TUBULAR STRUCTURE COMPOSED AT LEAST PARTIALLY OF INSULATING MATERIAL SLIDABLY DISPOSED IN THE BORE IN THE BUSHING AND CARRYING OTHER CONTACT MEANS AT ONE END THEREOF WHICH MAKES ELECTRICAL CONNECTION WITH THE CONTACT MEANS ON THE BUSHING WHILE THE TUBULAR STRUCTURE IS IN THE BORE IN THE BUSHING, THE TUBULAR STRUCTURE HAVING A PLURALITY OF THREADS ON THE OUTER SURFACE THEREOF AT THE OTHER END THEREOF ADAPTED TO BE ENGAGED BY SAID PAWL, CONNECTING ROD MEANS COMPOSED OF CONDUCTIVE MATERIAL SLIDABLY DISPOSED IN THE TUBULAR STRUCTURE, FIRST COUPLING MEANS ON THE CONNECTING ROD MEANS, THE TUBULAR STRUCTURE INCLUDING SECOND COUPLING MEANS, THE FIRST AND SECOND COUPLING MEANS ENGAGING EACH OTHER WHILE THE TUBULAR STRUCTURE AND CONNECTING ROD MEANS HAVE A PREDETERMINED AXIAL RELATIONSHIP WITH RESPECT TO EACH OTHER FOR TRANSMITTING ROTARY MOTION FROM THE CONNECTING ROD MEANS TO THE TUBULAR STRUCTURE, AND A FUSE ELEMENT EXTENDING BETWEEN THE OTHER CONTACT MEANS AND THE ADJACENT END OF THE CONNECTING ROD MEANS, SAID THREADS BEING SLIDABLE PAST THE PAWL WHEN THE TUBULAR STRUCTURE IS INSERTED IN THE BORE IN THE BUSHING, THE PAWL ENGAGING THE THREADS WHEN THE TUBULAR STRUCTURE IS BEING WITHDRAWN FROM THE BORE IN THE BUSHING WHEREBY THE TUBULAR STRUCTURE MUST BE TURNED TO PERMIT MOVEMENT OF THE THREADS PAST THE PAWL. 